The present invention relates to a system for measuring the pressure sealed inside an envelope of a lamp and, more particularly, to a system for optically measuring the pressure of a gas sealed in a lamp without requiring destruction of the lamp.
An inert gas such as argon, xenon and krypton is filled to a predetermined pressure inside a lamp. When the pressure of the gas sealed in the lamp is higher than a predetermined pressure, the lamp may break down during operation. On the other hand, when the pressure of the gas is lower than the predetermined pressure, the life time of the lamp is shortened. Therefore, it is necessary to check the pressure of the gas in the lamps after manufacturing them. As a method for measuring the pressure of the gas in the lamp, the so-called destructive inspection method is known. According to this destructive inspection method, a lamp is destroyed in water, the gas inside the lamp is collected under atmospheric pressure, and the volume of the gas is measured. The internal volume of the lamp is measured by another method. The pressure of the gas in units of atmospheres is calculated from the ratio of the internal volume of the lamp to the volume of the gas. Since the lamp is destroyed in this destructive inspection method, this method cannot be applied to all the lamps which are manufactured. For this reason, only some lamps are sampled and the gas pressure of the other lamps is assumed statistically from the gas pressure of the sampled lamps. Especially, all the lamps that are made by the existing manufacturing lines cannot have the same gas pressure desired. Some lamps having a gas pressure much different from the predetermined pressure may be manufactured. In the past, there have been cases wherein lamps sealed with gas under abnormal pressures were manufactured, which resulted in serious accidents.
In this way, according to the destructive inspection method, lamps of abnormal gas pressure alone cannot be segregated, and the conventional pressure test cannot be performed for all the lamps manufactured.